This invention is related to earlier inventions, one of which is entitled "Dual Frequency Surface Coils" the other of which is entitled "Multiple Frequency Surface Probes." The above Patent Applications are assigned to the Assignee of this inventions. The first of above-listed invention was described in a Patent Application filed in the U.S. on Mar. 19, 1985, and received Ser. No. 713,689. It issued on Sept. 1, 1987, as U.S. Pat. No. 4,691,163. The second above-listed Patent Application was filed in the United States on July 29, 1987, and received Ser. No. 78,895. It issued Dec. 20, 1988, as U.S. Pat. No. 4,792,759.
In MR data acquisition systems, radio frequency (RF) coils or probes are used to transmit RF pulses which perturb or tip onto a transverse plane, nuclei ("spins") that have magnetically aligned along a large static magnetic field. After the RF pulses are removed, the perturbed nuclei tend to dephase and to revert to the former aligned positions. The movement of the nuclei in the transverse plane creates detectable signals known as free induction decay (FID) signals.
When combinations of RF and gradient pulses are applied the dephasing nuclei are rephased and RF echo signals are generated. The FID and/or echo signals are detected by RF coils to provide data used for generating display images.
The most commonly perturbed element in magnetic resonance imaging (MRI) is hydrogen. Other elements are also perturbed, such as, for example, sodium. Also, for a long time it has been known that phosphor can also provide unique information when subjected to large static magnetic fields and perturbed by RF pulses. Thus, phosphor has been perturbed to obtain magnetic resonance spectrographic data.
Data signals are created by the nuclei which when dephasing cut magnetic lines of force. The RF pulses are applied using RF coils or probes. The signals are detected either using the same coils or separate receiving coils especially designed for detecting the weak signals. Because the signals are so weak everything that is possible is done to enhance the signals to enable obtaining images and/or spectrographic information from the signals.
For a long time MRI data was acquired using body coils; that is, coils large enough to receive the patient. Subsequently, it has been found that the use of surface coils applied at the surface of the patient's skin and relatively close to the portion of the patient being imaged enhances the acquisition of the signals needed for data acquisition purposes. The surface coils are in addition to the regular RF body coil built into the magnet along with gradient coils and the coil which generates the large static magnetic field.
The surface coils have to be matched to the system impedance and tuned to the appropriate Larmor frequency which is a function the type of nuclei from which data is acquired. To properly match surface coils requires components such as resistors and capacitors. To properly tune the surface coil also requires capacitance usually in the form of capacitors. Thus to match and/or tune the surface coils additional components have to be coupled to the coils. The additional components increase the minimum size of the surface coils even when the components are miniaturized.
In instances when the surface coils are used to obtain data at more than one Larmor frequency even more components have to be added to the surface coil to tune it to more than one frequency and accordingly the size of the surface coil increases and its reliability decreases.
It is desirable that surface coils be as small as possible to focus the transmitted radio frequency pulses at the portion of the patient from which data acquisition is desired and to receive signals primarily from a selected portion of the patient juxtaposed to the surface coil. Thus, there are the following seemingly conflicting goals for the idealized surface probe:
(1) it is desirable that the surface probe be as small as possible; and PA0 (2) it is desirable that the surface probe be properly matched and tuned.
The conflict of goals is further amplified when the surface probe is the type used to receive data at more than one frequency.
Accordingly, there is a long felt need for miniaturized surface probes that can be properly matched and tuned and although small in size still suffice to detect the small signals of MR systems at more than one frequency.